The present invention relates to a proton conductive solid electrolyte with high ion conductivity or a hydroxide ion conductive solid electrolyte with high ion conductivity that is applicable to a fuel cell or the like, and an electrochemical system using the solid electrolyte with high ion conductivity.
Conventionally, electrochemical systems, such as a fuel cell, a dehumidifier, or an electrolytic hydrogen producing device are commercialized as an electrochemical system using a proton conductive solid electrolyte. More specifically, a proton conductive solid electrolyte film, which operates at a low temperature, covers a broad range in application. For example, in a solid polymer fuel cell, current flows and electric energy is obtained in accordance with an electrochemical oxidative reaction of hydrogen supplied to a negative electrode that is indicated by the following formula (1) , an electrochemical reduction of oxygen supplied to a positive electrode that is indicated by a formula (2), and a reaction based on proton motion in the electrolyte between the positive electrode and the negative electrode.H2→2H++2e−  (1)1/2O2+2H++2e−→H2O  (2)
It is known a fuel cell of direct methanol type in which a methanol is supplied as a fuel to the negative electrode of the fuel cell. Furthermore, it is known a fuel cell using another fuel instead of hydrogen or the methanol. Also in these cases, the reaction is carried out in which the fuel is electrochemically oxidized at the negative electrode to release proton, in a similar manner. Therefore, it is possible to operate by using the proton conductive solid electrolyte.
For example of the electrolytic device, the electrolytic hydrogen producing device is commercialized. The electrolytic hydrogen producing device produces hydrogen on the basis of a reaction inverse to the reaction described in conjunction with the formulas (1) and (2) in the fuel cell. Inasmuch as it is possible to obtain high purity hydrogen in on-site by using only water and electric power in the electrolytic hydrogen producing device, it is unnecessary to have a hydrogen gas cylinder. In addition, it is possible to easily carry out electrolysis by introduction of pure water having no electrolyte solute, owing to employ the solid electrolyte. Using a similar system, an attempt is made to manufacture hydrogen peroxide for bleach by the electrolytic method using the following formula (3), in paper industry (referring to a non-patent publication 1).O2+H2O+2e−→HO2−+OH−  (3)
The dehumidifier has a structure in which the proton conductive solid electrolyte is sandwiched between the positive electrode and the negative electrode, in a manner similar to the fuel cell or the hydrogen producing device. When a voltage is applied between the positive electrode and the negative electrode, water is split into proton and oxygen at the positive electrode on the basis of the reaction indicated by the following formula (4). The proton moves through the solid electrolyte to the negative electrode to be subjected to a reaction indicated by a formula (5). As a result, the union of the proton and the oxygen in air forms water. As a result of these reactions, water moves from the positive electrode to the negative electrode so that dehumidification is carried out in the positive electrode.H2O→1/2O2+2H++2e−  (4)1/2O2+2H++2e−→H2O  (5)
It is also possible to split water and to eliminate moisture by using the principle of operation that is similar to the electrolytic hydrogen producing device. Proposal is made as regards an air conditioner in which the electrolytic hydrogen producing device is combined with a moisture evaporating cold blast device (referring to non-patent publication 2).
In any one of the above-mentioned systems, perfluoro sulfonic acid type ion exchange membrane represented by Nafion is used as the solid electrolyte. In addition, a sort of sensors, electrochromic device or the like is essentially a system based on a principal of operation similar to the above-mentioned. Inasmuch as the system is driven when the proton moves in the electrolyte between a pair of positive and negative electrodes which carry out reduction and oxidation, respectively, it is possible to use the proton conductive solid electrolyte. At present, experimental study is carried out with respect to the system using these proton conductive solid electrolytes.
For a hydrogen sensor, variation of electrode potential based on the concentration of hydrogen when hydrogen is introduced into the hydrogen sensor in the reaction indicated by the above-mentioned formulas (4) and (5) may be used. Furthermore, it is also possible to be applied to a humidity sensor, by using the variation of electrode potential or ion conductivity.
When an electric field is applied to the negative electrode of the electrochromic device using WO3 or the like, the electrochromic device makes a color on the basis of reaction indicated by the following formula (6) and can be used as a displaying device or a light proof glass. This system operates when protons are given to the negative electrode. It is possible to use the proton conductive solid electrolyte in this system.WO3+xH−+xe−→HxWO3 (coloring)  (6)
In addition, there are a primary battery, a secondary battery, an optical switch, and an electrolyzed water producing apparatus, as the electrochemical system which operates by using the proton conductive solid electrolyte in principal. For example, a hydrogen absorbing alloy is used as the negative electrode, a nickel hydroxide is used as the positive electrode, and an alkali electrolytic solution is used as the electrolytic solution in a nickel hydride battery of the secondary battery. As indicated by formulas (7) and (8), the electrochemical reduction and oxidation with respect to the proton and hydrogen absorption in the hydrogen absorbing alloy occur in the negative electrode on charge and discharge.(charge) H2O+e−→H(absorbing)+OH−  (7)(discharge) H(absorbing)+OH−→H2O+e−  (8)
As indicated by formulas (9) and (10), the electrochemical oxidation and reduction occur with respect to the nickel hydroxide in the positive electrode.(charge) Ni(OH)2+OH−→NiOOH+H2O+e−  (9)(discharge) NiOOH+H2O+e−→Ni(OH)2+OH−  (10)
The charge and discharge reaction occurs in the battery by the transfer of the proton or the hydroxyl ion in the electrolyte. Although it is possible to use the proton conductive solid electrolyte in principal, the alkali electrolytic solution is used in the prior art.
Proposal is made in the optical switch about using yttrium as the negative electrode (referring to non-patent publication 3). When supplied with the electric field, yttrium is hydrogenated as indicated by formula (11) to allow the light to pass therethrough. As a result, it is possible to switch between the light transmission and the non-light transmission by electric field. Although it is possible to use the proton conductive solid electrolyte in principal in this system, the alkali electrolytic solution is used in the prior art.Y+3/2H2O+3e→YH3+30H  (11)
The electrolyzed water is water which is produced by the electrolyzing reaction. Although availability is different between the reduction side and the oxidation side, the electrolyzed water has availability in a healthy effect, a bactericidal action, a detergent action, and a growth of farm products. It is possible to use the electrolyzed water in drinking water, food service water, detergent water, agricultural water or the like. The electrolyzing reaction is promoted when the water has the electrolyte. When the electrolyte solute is dissolved in water, it often necessary to remove the electrolyte solute from the water on using the water. When the solid electrolyte is used as the electrolyte, it is unnecessary to remove the electrolyte solute from the water.
The conventional proton conductive solid electrolyte for the low temperature operation, which is used in each of the above-mentioned electrochemical systems, is almost a polymeric ion exchange membrane of perfluoro sulfonic acid type that is represented by Nafion film. However, there is a problem in which the perfluoro sulfonic acid type electrolyte is expensive on the basis of complexity of manufacturing process. By the economies of mass production, it is expected that a low-priced electrolyte is manufactured. However, there is limitation of the low-price. It is desired that a cheap alternate member appears presently. In addition, an amount of methanol used as the fuel permeates through perfluoro sulfonic acid type electrolyte, in the direct methanol type fuel cell. As a result, there is a problem in which energy conversion efficiency greatly reduces.
By the way, proposal is made as regards a complex compound having polyvinyl alcohol and various inorganic compounds, as cheep high ion conductive electrolytic material instead of the perfluoro sulfonic acid type electrolyte. For example, the complex compound is proposed which is obtained by mainly chemically bonding polyvinyl alcohol to silicic acid compound in a micro-level (referring to patent publication 1). In addition, the complex compound is proposed which is obtained by mainly chemically bonding polyvinyl alcohol to tungstic acid compound in a micro-level (referring to patent publication 2). The complex compound is further proposed which is obtained by mainly chemically bonding polyvinyl alcohol to molybdic acid compound in a micro-level (referring to patent publication 2). The complex compound is further proposed which is obtained by mainly chemically bonding polyvinyl alcohol to stannic acid compound in a micro-level (referring to patent publication 3). The complex compound is further proposed which is obtained by mainly chemically bonding polyvinyl alcohol to zirconic acid compound in a micro-level (referring to patent publication 4 or 5). The complex compound includes at least one selected from phosphorus, boron, aluminum, titanium, calcium, strontium, and barium compound, as other components. It is possible to produce the complex compound when a simple process is carried out which neutralizes the raw salt of inorganic compound in a solution with the polyvinyl alcohol coexisting. The proton conductivity is given together with water resistance and strength to the polyvinyl alcohol by chemically bonding the polyvinyl alcohol to the inorganic compound. Flexibility is given to the inorganic compound by chemically bonding the polyvinyl alcohol to the inorganic compound. As a result, it is possible to manufacture the solid electrolyte having a high performance.
In addition, the above-mentioned complex compound is different from the conventional solid electrolyte of the perfluoro sulfonic acid type and has a high ion conductivity in an alkaline form. It is possible to apply to a primary battery, a secondary battery, an optical switch, or the like which is difficult to use the conventional solid electrolyte. Furthermore, by developing an alkaline form solid electrolytic film, it is easy to realize the secondary battery with a high energy density using a multivalent metal having an oxidation number which is not less than bivalent. For example, a nickel zinc cell is known in which a zinc oxide is used in the negative electrode and a nickel hydroxide, which is used also in a nickel hydride cell, is used in the positive electrode. The nickel hydroxide is used in a nickel hydride cell. In the nickel zinc cell, the zinc oxide is reduced into the zinc in the negative electrode at charge, as indicated by a formula (12). On the other hand, the zinc is electrochemically oxidized into the zinc oxide in the negative electrode at discharge, as indicated by a formula (13).(charge) ZnO+H2O+2e−→Zn2OH−  (12)(discharge) Zn+2OH−→ZnO+H2O+2e−  (13)
Although the nickel zinc cell has a high stored energy inasmuch as the zinc has bivalent, there is a problem on which it is difficult to realize the nickel zinc cell because of dissolution of the zinc oxide and production of a needle shaped zinc, such as dendrite, which induces a short-circuit and a self-discharge. However, it is possible to resolve the problem by using the solid electrolyte. In addition, oxygen is restricted from diffusing to a zinc electrode in air zinc cell using an air electrode as a positive electrode. As a result, it is possible to easily obtain a chargeable air zinc cell. Furthermore, it is possible to realize the secondary cell using selected ones of the multivalent metals, by the solid electrolyte, inasmuch as there are copper, cobalt, iron, manganese, chromium, vanadium, tin, molybdenum, niobium, tungsten, silicon, boron, and aluminum as the multivalent metals except for zinc.
In other uses except for cell, the material, which is capable of being used as an electrode or a peripheral material, is not limited to an acid-proof material such as a noble metal, in case of using the alkaline form solid electrolyte. As a result, there is a merit in reducing a cost in an entire system.                (Patent Publication 1)        Japanese Unexamined Patent Publication Tokkai 2003-007133        (Patent Publication 2)        Japanese Unexamined Patent Publication Tokkai 2001-335314        (Patent Publication 3)        Japanese Unexamined Patent Publication Tokkai 2002-4151        (Patent Publication 4)        Japanese Unexamined Patent Publication Tokkai 2002-35832        (Patent Publication 5)        Japanese Unexamined Patent Publication Tokkai 2002-310093        (Non-patent Publication 1)        Electrochemistry, 69, No.3, 154-159(2001)        (Non-patent Publication 2)        
Collected papers of national lecture in Institute of Electrical Engineers, P3373(2000)                (Non-patent Publication 3)        J.Electrochem.Soc., Vol. 143, No.10, 3348-3353 (1996)        
Although there are many merits in applying to a wide range of uses inasmuch as the solid electrolyte produced by the polyvinyl alcohol and the inorganic compound is cheap and has a high efficiency, there is a problem in swelling greatly on the basis of water absorption in case of positioning the solid electrolyte in a wet condition, and reducing the strength of the solid electrolyte when the solid electrolyte swells. More particularly, the swelling is an important problem inasmuch as the solid electrolyte is positioned in the wet condition or the solid electrolyte is directly immersed in water, in case where the solid electrolyte is used in a fuel cell, an electrolysis device, or the like. Even in other uses except for the fuel cell or the electrolysis device, there is a problem in varying a size of the solid electrolyte with humidity. In addition, there is a problem in reducing an energy efficiency inasmuch as the methanol of fuel substantially permeates through the solid electrolyte composed of the complex compound, in use of the direct methanol type fuel cell, although the permeability is lower in comparison to the perfluoro sulfonic acid type electrolyte.